Conventionally, as a sliding bearing having a high rotational accuracy, a sliding bearing composed of a porous sintered metal impregnated with oil is known. When this sliding bearing is used by impregnating a porous material consisting of the sintered metal with the oil, it is possible to successively supplying a sliding interface with the oil. Thus it is possible to reduce a frictional force. Because a mating material of the sliding bearing is generally made of the same metal material as that of the sliding bearing, there is no fear of wrap of the sliding bearing around the mating material and separation of the sliding bearing therefrom owing to the difference between the linear expansion degree of the mating material and that of the sliding bearing. Because the processing accuracy of this metal material can be enhanced, the metal material is suitably used at a position where a high rotational accuracy is demanded.
Other than the above-described sliding bearing, a sliding bearing, having self-lubricating properties, which is composed of a resin to which a solid lubricant such as polytetrafluoroethylene, graphite, molybdenum disulfide or the like, a lubricating oil or wax is added is known.
But there is a fear that the sliding bearing composed of the porous sintered metal impregnated with the oil wears the mating material made of a soft material. Further when the supply of the lubricating oil is interrupted, there is a fear that metal contact occurs. On the other hand, when a resin material having an excellent sliding property is used for the sliding bearing, the resin material does not attack the mating material made of the soft material. But in this case, a problem that owing to contraction and expansion of the resin material, wrap of the sliding bearing around the shaft occurs. Further in this case, it is necessary to increase the gap between the bearing and the shaft. Thereby a problem that the rotational accuracy deteriorates occurs.
Therefore the following high-accuracy sliding bearing is disclosed (see patent document 1): A metal is used for a bearing peripheral part of the sliding bearing. A resin material is molded at a sliding portion of the bearing peripheral part to form a resin layer. Fine concavities are formed essentially at a portion, of the surface of the bearing peripheral part, which contacts the resin layer. In the resin layer, (the coefficient of linear thermal expansion of the resin material)×(the thickness of the resin layer) is set to not more than 0.15. The apparent area of the concavities is set to 25 to 95% of the area of the portion, of the surface of the bearing peripheral part, which contacts the resin layer (see patent document 1).
Because a dimensional change of the resin layer caused by a change of temperature is suppressed, the above-described high-accuracy sliding bearing has a high accuracy and an excellent lubricating performance. Further because the high-accuracy sliding bearing has the resin layer formed on the sliding portion thereof, the high-accuracy sliding bearing does not attack a mating material made of a soft material nor generates an abnormal noise. Furthermore the high-accuracy sliding bearing has another characteristic that the resin layer adheres strongly to the bearing peripheral part by an anchor effect because the resin layer penetrates into the fine concavities.
But as the accuracy of the high-accuracy sliding bearing becomes higher, a gate mark formed at a time of injection molding may adversely affect the performance of the bearing. In addition, there is an increase in the number of producing steps owing to treatment of the gate mark. Thus the high-accuracy sliding bearing may have an inferior productivity.
Patent document 1: Japanese Patent Application Laid-Open No. 2003-239976